In recent years, attention has been drawn to optical communication as means for coping with a rapidly increasing demand for communication. In this connection, a WDM optical communication system has attracted attention, because this system can transmit optical signals with different wavelengths over a single optical fiber to realize high capacity communication over a single optical fiber. WDM is a system wherein, on a transmitter side, a plurality of different wavelengths (n wavelengths: λ1 to λn) are multiplexed in an optical multiplexer to perform wavelength multiplexing on a single optical fiber, followed by transmission, while on a receiver side, information from the optical fiber is demultiplexed in an optical demultiplexer to wavelengths λ1 to λn. The realization of WDM requires, for example, the stabilization of laser wavelengths, the development of optical circuit devices, and the integration of optical circuits. Further, in the WDM optical communication system, a plurality of light sources corresponding respectively to the wavelengths λ1 to λn are required. Accordingly, what is required here is to efficiently realize light sources respectively with different wavelengths. To cope with this, Japanese Patent Laid-Open No. 117040/1998 discloses a production process wherein a plurality of DFB (distributed feedback) lasers with different wavelengths and a plurality of EA (electro-absorption) modulators, which have been integrated with each other, are simultaneously prepared within a plane of a single semiconductor substrate. According to the production process disclosed in Japanese Patent Laid-Open No. 117040/1998, in order to provide a plurality of different oscillation wavelengths on a single semiconductor substrate, diffraction gratings with different periods (pitches Λ1, Λ2 . . . Λn) are formed by electron beam exposure and etching, and multi-layer structures each comprising an active layer (a light absorption layer) with a band gap wavelength according to the oscillation wavelength are then prepared by selective MOVPE (metal-organic vapor phase epitaxy).
FIG. 33 shows an optical semiconductor device disclosed in Japanese Patent Laid-Open No. 117040/1998. As shown in FIG. 33A, at the outset, 16 phase-shift diffraction gratings 302 with different periods are successively formed on an n-type InP substrate 301 in its stripe region by electron beam (EB) exposure and etching. The pitch of the diffraction gratings 302 is varied to Wm1, Wm2, and Wm3 in the order of an increase in the pitch. Next, a striped SiO2 mask 303 as an insulation layer for selective growth is formed in the direction of [011]. This SiO2 mask 303 is formed on the diffraction grating 302 so as to have a striped window region having a predetermined width. In this case, the mask width is varied according to the pitch of the diffraction grating 302. Thereafter, as shown in FIG. 33B, an n-type InGaAsP guide layer 304, an InGaAsP/InGaAsP-MQW (multiple quantum well) active layer 305, and a p-type InP cladding 306 are formed in that order by selective MOVPE growth in a region between the SiO2 masks 303. The multilayer semiconductor layer formed in the region between the SiO2 masks 303 functions as an optical waveguide. In FIG. 33B, a semiconductor layer formed between masks with different widths is not shown in the drawing for the simplification of the structure.
Next, the opening width of the SiO2 mask 303 around the striped optical waveguide is widened. Thereafter, as shown in FIG. 33C, a p-type InP buried layer 307 is formed again by selective MOVPE growth. In FIG. 33C, as with FIG. 33B, the semiconductor layer formed between the SiO2 masks 3 with different widths is not shown in the drawing.
Next, an SiO2 layer 308 is formed on the assembly except for the top of the ridge structural multilayer semiconductor including the MQW active layer 305, and, as shown in FIG. 33D, a metal electrode 309 is formed on the surface of the InP substrate 301, while a metal electrode 310 is formed on the backside of the InP substrate 301. The metal electrode 309 formed on the surface of the InP substrate 301 is separated for insulation between the devices. Thereafter, the wafer is cleaved at an interval of the semiconductor laser device length. An antireflection (AR) coating is applied onto the cleaved end face to complete semiconductor lasers.
According to the process shown in FIG. 33, the band gap wavelength of the oscillation wavelength can be made identical to the band gap wavelength of the laser active layer in a given range (detuning). Therefore, this process features that the homogeneity of the threshold of laser oscillation and the oscillation efficiency can be kept relatively good. The preparation of the active layer by selective MOVPE, however, leads to a change in band gap wavelength of the optical guide layer formed on the diffraction grating and, in addition, a change in thickness of the active layer. The change in the band gap wavelength of the guide layer leads to a change in absolute value of the refractive index on the diffraction grating. This in turn leads to a change in level of a periodic refractive index change by diffraction gratings. Further, a change in thickness of the active layer leads to a change in an optical confinement factor in the active layer. This results in a change in light intensity in the diffraction grating region. The level of the periodic refractive index change due to the diffraction grating and the light intensity of the diffraction grating region are parameters involved directly in coupling coefficient κ (which represents the relationship of coupling where reflection occurs in the diffraction grating to couple a traveling wave with a back wave and is a parameter as an index for resonant characteristics). Therefore, in the simultaneous formation of lasers with different wavelengths wherein the level of the refractive index change and the light intensity in the diffraction grating region are varied, the coupling coefficient κ is varied according to the oscillation wavelength.
Here an optical semiconductor device produced by the production process shown in FIG. 33 will be discussed. (1) The increase in the widths (Wmm1 to Wmm3) of the masks for selective growth to increase the oscillation wavelength increases the band gap wavelength of the optical guide layer on the diffraction grating. This increases the absolute value of the refractive index of the optical guide layer. As a result, the coupling coefficient κ is increased. (2) Since the thickness of the active layer is increased, the coefficient of light confining in the active layer is increased. Thus, the light intensity in the diffraction grating region is reduced, and the coupling coefficient κ is reduced. The relationship between the oscillation wavelength (or the width of mask for selective growth) and the coupling coefficient κ varies depending upon the relationship between the magnitude in (1) and the magnitude in (2) (which depends upon an MOVPE apparatus for growth of crystal or growth conditions). This coupling coefficient κ is a parameter which is closely related, for example, to the oscillation threshold of DFB laser, luminous efficiency, yield of longitudinal single mode, and long-distance transmission characteristics.
According to the conventional optical semiconductor device and production process, however, since the coupling coefficient κ varies for each DFB oscillation wavelength, the threshold current of laser oscillation and the luminous efficiency become heterogeneous. This heterogeneity results in lowered yield of devices.
Further, since the coupling coefficient κ varies for each DFB, the resistance to the residual reflection of the end face varies from device to device. This poses a problem that, in the case of long-distance transmission, the yield of transmission characteristics varies according to the wavelength.